GEOPHYSICS

The most deadly earthquake of 1995, having a magnitude of 7.2, struck January 17 in the vicinity of Kobe, Japan. Named the Great Hanshin Earthquake, it killed some 6,000 persons and injured more than 30,000. Nearly 200,000 buildings were destroyed or seriously damaged, and more than 300,000 people had to be housed in temporary shelters. Ground effects included liquefaction of the surface in the vicinity of the epicentre and a nine-kilometre surface fracture, with horizontal displacements reaching 1.5 m. (One kilometre is about 0.62 mi; one metre is about 3.3 ft.) Another high-fatality earthquake, having a magnitude of 7.5, occurred May 28 in and around the town of Neftegorsk, Sakhalin Island, in the Sea of Okhotsk off eastern Russia; nearly 2,000 people lost their lives.

Scientists from Oregon State University mapped a blind thrust fault in Ventura county, Calif. The structure, named the Oak Ridge Fault, was designated as blind because it does not reach the surface but is overlaid by the Santa Susana thrust fault. It is the site of the Jan. 17, 1994, Northridge earthquake, which caused more than 60 deaths and major destruction throughout the stricken area. During the Northridge quake both sides of the Santa Susana Fault were displaced owing to the movement on the fault hidden beneath it. It was postulated that if a fault runs through the mountains, rather than along the edge of a valley, as is the case with the Santa Susana Fault, then it is probable that a blind fault lies beneath it.

The physical mechanism by which energy is suddenly released in deep-focus earthquakes--i.e., those that occur below about 400 km depth--has long been a puzzle to seismologists. At such depths high temperature and pressure should cause rock under stress to flow smoothly rather than rupture suddenly, as it does in earthquakes near the surface. Recent studies by researchers at the University of California, Santa Cruz, showed that on average the deeper the focus, the more symmetrical the pattern of energy release over time. As recorded on a seismograph, the disturbances caused by a deep-focus earthquake tend to begin abruptly, build to a maximum, and then end relatively quickly and smoothly. The researchers believed that such a pattern is due to the uniformity of the material at the focus but could not determine whether it is the result of a rupture or a geochemical transformation that releases a burst of energy.

A strong impetus to the search for an acceptable theory for deep-focus earthquakes resulted from the occurrence of the great Bolivian earthquake of June 9, 1994. At magnitude 8.2 it was the largest shock on record to have had a focus more than 600 km below the surface, at the base of the upper mantle. Upon analysis by investigators of the Carnegie Institution of Washington, D.C., and the University of Arizona, the rupture zone was found to be many times too large--it covered a horizontal area 30× 50 km--to fit the currently accepted olivine-spinel transformation theory. According to that explanation, transformation under pressure of the mineral olivine into a more stable mineral, spinel, causes microfissures, which permit an earthquake to occur. Because deep-focus earthquakes generally take place beneath areas of active subduction, where the edge of one of a pair of colliding crustal plates is descending beneath the edge of the other plate, it was thought that such quakes have their origin in subducted crustal slabs that have survived the descent to deep-focus depths. Because the slab supposedly erodes and thins as it descends, however, at 600 km or deeper it would be much thinner than the size of the fracture zone calculated for the Bolivian earthquake. Several studies were under way to test various alternative theories. One speculative idea was that under the extremes of temperature and pressure at depth, some kind of nuclear reaction occurs that releases energy directly, with little or no physical deformation.

As was happening in other spheres of science, geophysics was benefiting greatly from high technology. Developments in computers and instrumentation were increasing accuracies and resolution manyfold. Two techniques for exploring beneath the Earth’s surface recently gained recognition. One, called cross-borehole seismology, was first used by scientists at the French Petroleum Institute in the early 1970s but did not attain wider acceptance until advances in instrumentation made it feasible. Seismic studies on the surface collect data on wavelengths of 20-100 m, while well logs (records made during well drilling) register wavelengths of 0.3-1 m and measure the environment immediately around the borehole. In contrast, cross-borehole seismology covers the range of wavelengths from two to five metres. Instruments are set up in an array, with receivers vertically spaced in one borehole and signal generators placed in surrounding boreholes at distances of 100-300 m. The generated signals are tailored so as not to damage the borehole but still be strong enough for reception. By means of multiple receivers and multistation receiver cables, it is possible to record as many as 25,000 seismograms in a few days. The analysis of the data is quite complex, combining the techniques of medical X-ray computed tomography and more conventional wave-tracing techniques of exploration seismology with enhancement from standard reflection imaging. The dramatic enhancement of rock-structure definition gained by the technique was expected to increase the detection of high-porosity zones and permeability barriers and thus help identify oil reservoirs and their dimensions.

The second technique, geophysical diffraction tomography, is similarly derived from medical tomography. First developed in the early 1980s, it involves the mathematical combination of many individual signals from a specifically designed array of instruments to produce a three-dimensional image of the region traversed by the signals. As of 1995 it had been used to detect underground tunnels across the demilitarized zone between North Korea and South Korea; to trace the outline of the still unexcavated fossil bones of Seismosaurus, an enormous dinosaur discovered in the southwestern U.S.; and to map the remains of ancient underground settlements in the Negev region of Israel.

Using data collected by satellites of the Global Positioning System (GPS), researchers from the University of Colorado and Stanford University found that Australia is moving north-northeast with respect to Antarctica at a rate of five to eight centimetres (two to three inches) per year. The detection of that heretofore unknown movement was made possible by means of weekly measurements of the relative positions of points all over Antarctica, Australia, Hawaii, New Zealand, Tahiti, and Tasmania carried out by GPS satellites and disseminated on the Internet. The GPS system was capable of measuring positional variations of less than 2 mm (0.08 in).

Work carried out on Legs 152 through 158 of the International Ocean Drilling Program (ODP), which studied the crust beneath the world’s oceans by means of the coring and extraction of rock samples from below the seafloor, was confined to the Atlantic Ocean. Exploration proceeded from sites on or near the continental shelf southeast of Greenland (Leg 152) to the Mid-Atlantic Ridge south of the Kane Fracture Zone (Leg 153), to a transept across the Ceara Rise in the western equatorial Atlantic (Leg 154), to the Amazon River deep-sea fan (Leg 155), to the deformation front of the North Barbados Ridge (Leg 156), to the Canary Basin (Leg 157), and finally to the Mid-Atlantic Ridge at latitude 26° N (Leg 158). The ODP expeditions collected data relevant to paleoceanography (study of the ocean in past ages), seafloor spreading, and the evolution of the Mid-Atlantic Ridge at those critical sites.

Earth and Space Sciences: Year In Review 1995. (2015). In Encyclopædia Britannica. Retrieved from http://www.britannica.com/EBchecked/topic/176126/Earth-and-Space-Sciences-Year-In-Review-1995/232990/GEOPHYSICS

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